This invention relates to methods and devices for calcining gypsum material, and more particularly, to a method and system for calcining gypsum to recover calcium sulfate anhydrite which is often referred to, in insoluble form, as "dead burn".
Many methods and devices for calcining gypsum are well known. Traditionally, refractories have been used to calcine gypsum in large kettles having a thickened dome-shaped bottom, and the kettle is heated by gas-fired flames in a brick refractory structure. (See U.S. Pat. No. 3,236,509). However, using a refractory to calcine gypsum results in an extreme waste of energy due to the excessive amount of heat which must be applied to the kettle to heat the gypsum contained therein, and the refractory brick enclosure is also inefficient since it has a large footprint and occupies valuable factory space. Other calcining methods and devices have taken the form of refractoryless kettles which use submerged combustion heating systems within the kettle such as disclosed in U.S. Pat. Nos. 4,626,199, 4,629,419 and 4,744,961. One major objective of both refractory and refractoryless kettles for calcining gypsum has been to produce calcium sulfate hemihydrate, better known as stucco, for use in the production of wallboard.
In contrast to such prior art methods which were concerned with production of hemihydrate or stucco, the method and system of this invention are concerned primarily with recovering gypsum material from the calcining process which consists essentially of calcium sulfate anhydrite. The recovered anhydrite product may be in the form of soluble calcium sulfate anhydrite which is slightly unstable or more preferably is recovered in the form of insoluble calcium sulfate anhydrite which is stable and often referred to as dead burn. Dead burn material has many applications including as a filler in thermoplastics, herbicides, foods and pharmaceuticals, cement, plaster additives, etc. However, the production of dead burn is difficult.
Raw gypsum is generally in the form of a dry powder and takes the form of CaSO.sub.4.2H.sub.2 O. When raw gypsum is heated to a temperature of generally about 250.degree. F.-380.degree. F. or even higher, the powder converts to hemihydrate which takes the form of CaSO.sub.4.1/2H.sub.2 O+11/2H.sub.2 O. The 11/2H.sub.2 O is in the form of water vapor and fluidizes the dry powder during the calcining process so that it will flow through the apparatus. When the hemihydrate is heated to even higher temperatures, the gypsum converts to soluble anhydrite or insoluble anhydrite CaSo.sub.4 (dead burn). However, the 1/2H.sub.2 O released during conversion to soluble or insoluble anhydrite does not fluidize the powder very well.
Due to such fluidization problems, conventional refractory or refractoryless calcining methods to produce dead burn have been less than effective in efficiently and economically producing dead burn. Specifically, using a refractory for producing dead burn material is excessively expensive and cost prohibitive due to required temperatures of about 900.degree. F. to 1300.degree. F. The prior art refractoryless methods of calcining gypsum are more economical than using a refractory but are impractical for producing dead burn material due to fluidization problems. In addition, the anhydrite produced from such systems is often not evenly heated and still contains substantial amounts of chemically-combined water which make the anhydrite unsuitable as a filler for thermoplastics and other similar applications. Other methods of calcining gypsum to produce dead burn have included using flash calciners to produce dead burn. Such flash calciners entrain the ground gypsum in a stream of accelerated air which is then flash heated to elevated temperatures. However, such systems are limited in their capacity and flowthrough rate.
An important aspect of this invention therefore lies in providing a method for producing dead burn material in a cost effective and efficient manner which avoids the excess cost of a refractory and overcomes the fluidization problems which would otherwise occur in using prior art methods of calcining gypsum to produce dead burn. Such a method involves feeding the gypsum material through two or more stages of calcining to gradually convert the gypsum to dead burn material. In the first stage, the gypsum material contains chemically-combined water which is released by the heating process to self-fluidize the gypsum powder so that it will flow through the apparatus. The gypsum powder in the first stage is generally heated to form a hemihydrate product which occurs in a temperature range of about 250.degree. F.-380.degree. F., or generally less than 400.degree. F. The material is then passed through at least one subsequent stage, preferably two or more stages, so that it is heated sufficiently to form calcium sulfate anhydrite. In the subsequent stages, the method includes the steps of heating and simultaneously fluidizing the material with a fluidization media, preferably air, so that it will flow through the subsequent stages of the system. The material is then recovered from the process in a form consisting essentially of calcium sulfate anhydrite. The recovered anhydrite product may be soluble or insoluble depending upon the desired application, and the insoluble anhydrite is generally referred to as dead burn material. For purposes of convenience, the term gypsum is generally used herein to describe the various forms of calcium sulfate, including dihydrate (gypsum), hemihydrate (stucco) and anhydrite (dead burn).
In one preferred embodiment of the method of this invention, the method involves three steps of calcining the gypsum through three calcining kettles to recover calcium sulfate anhydrite from the third kettle. In particular, the method comprises the steps of first feeding ground gypsum into a first kettle and heating the gypsum to a first predetermined temperature of about 250.degree. F.-380.degree. F., preferably about 310.degree. F. The gypsum powder is in a hemihydrate state at such a temperature and the release of water vapor, by the reaction CaSO.sub.4.2H.sub.2 O.fwdarw.CaSO.sub.4.1/2H.sub.2 O+11/2H.sub.2 O, sufficiently fluidizes the powder so that it will flow through the process. The next step is to overflow the heated gypsum from the first kettle into a second kettle. The material in the second kettle is then heated to a second predetermined temperature and simultaneously fluidized with a fluidizing media, preferably air, in the second kettle. The second predetermined temperature is about 500.degree. F.-800.degree. F., preferably about 600.degree. F. At such a temperature, the gypsum material will be a multi-phase. material having relatively poor flow characteristics. However, the fluidization of the gypsum powder with a fluidizing media in the second kettle ensures that it will properly flow through the system. The gypsum powder is then overflowed from the second kettle into a third kettle where it is then heated to a third predetermined or final temperature and simultaneously fluidized with a fluidizing media. The gypsum powder is then recovered from the third kettle as a gypsum material consisting essentially of calcium sulfate anhydrite. In a method to produce insoluble calcium sulfate anhydrite or dead burn, the third predetermined or final temperature should be greater than about 900.degree. F., preferably greater than about 930.degree. F., to ensure the production of dead burn material. Generally, the third predetermined temperature should be in the range of about 900.degree. F.-13000 F., and in one embodiment of the invention, the third predetermined temperature is about 1000.degree. F.
The step of fluidizing the gypsum powder in the second and third kettles includes providing a fluidization means in those kettles for fluidizing the gypsum material contained therein. In a preferred embodiment, the fluidization means includes a plurality of mixing blades and air pipes provided along breaking edges of the mixing blades for injecting air through a plurality of radially-directed ports into the contents of the second and third kettles. Preferably, the mixing blades include a pair of horizontally-extending blades, and a pair of oppositely-oriented, helically-twisted blades which extend vertically between the first and second horizontal blades. The helical blades each include a leading edge, a trailing edge and a breaking edge, and the air pipes are positioned along the breaking edges with the injection ports directed towards the burner coils. The blades are preferably arranged around a central axle, and an air source may be connected to the central axle which is then connected with the air pipes along the leading edges of the helical blades.
In the preferred embodiment, the fluidization means also includes a plurality of radially-arranged air injection nozzles positioned about a periphery of the kettle shells of the second and third kettles. Each air injection nozzle includes a plurality of air injection ports, and the nozzles are each connected to a pressurized air line for injecting pressurized air through the ports and into the contents of the kettle. The pressurized air sufficiently fluidizes the ground gypsum at elevated temperatures so that it will adequately flow through the system.
In an alternate construction, the fluidization means may include a perforated screen and a woven web or mat positioned in the bottom of the second and third kettles and a pressurized air chamber located below the screen and mat for injecting air through the screen and mat and into the interior of the kettle. Air is blown into the air chamber and through the screen and mat throughout the gypsum material at elevated temperatures so that the ground gypsum material will flow through the second and third stages of the apparatus.
The fluidization means is preferably formed of the combination of the helical mixing blades and air pipes along the leading edges of those blades as well as a plurality of radially-arranged air injection nozzles. However, the air injection nozzles may be replaced with use of an air chamber and perforated sheet and web in the bottom of the kettles. The fluidization means may also take a variety of other forms of means for fluidizing or aerating the gypsum powder in the kettles during the second and third stages when the gypsum powder does not sufficiently self-fluidize.
Other objects, features, and advantages will become apparent from the following description and drawings.